On_optimizing_images.md

JPG

mozjpeg allows optimizing the compression, losslessly recompressing an image.

mozjpeg input.jpg > output.jpg

Depending on how it's installed, the executable might be called jpegtran instead of mozjpeg. To convert other formats to jpg, use cjpeg, or the convert command from imagemagick, followed by mozjpeg.

There is also guetzli, but I never got good results with it.

SVG

The main minifier is svgo.

If the SVG image paths are breaking, turn off convertPathData and mergePaths.

If colors are turning black or being removed, turn off removeUnknownsAndDefaults.

There have also been reports of issues with collapseGroups, though I've never had an issue yet.

Usage example:

svgo --multipass --disable=removeUnknownsAndDefaults input.name -o output.name

Achtung: In some weird cases, --multipass actually compresses less than if not used.

PNG

For best results, use pngquant, followed by zopflipng.

pngquant for lossy compression

pngquant --speed 1 input.png

By default, the new image will be called input-fs8.png. If the image is very simple color-wise, we can try to really squeeze it using a low quality range, e.g.:

pngquant --speed 1 --quality=0-10 input.png

zopflipng for lossless compression

For the best results (although for most images it takes hours):

zopflipng --iterations=1000 --filters=01234mepb --lossy_8bit --lossy_transparent

GIF

gifsicle -O3 input.gif -o output.gif

General Script

#!/bin/bash

extension="${1##*.}"
backup=${1%${extension}}orig.${extension}
mv "$1" "${backup}"

optimize_svg() {
    # For some reason, sometimes not using --multipass compresses better
    # Since svgo is fast, we just try both and keep whichever is smaller.
    svgo --multipass --disable=removeUnknownsAndDefaults "${backup}" -o temp1.svg
    svgo --disable=removeUnknownsAndDefaults "${backup}" -o temp2.svg

    size1=$(stat -c %s temp1.svg)
    size2=$(stat -c %s temp2.svg)

    if [[ "$size1" -lt "$size2" ]]; then
        mv temp1.svg "$1"
        rm temp2.svg
    else 
        mv temp2.svg "$1"
        rm temp1.svg
    fi
}

case ${extension} in 
  "png")
    zopflipng --iterations=1000 --filters=01234mepb --lossy_8bit --lossy_transparent "${backup}" "$1"
    ;;
  "gif")
    gifsicle -O3 "${backup}" -o "$1"
    ;;
  "jpeg")
    ;&
  "jpg")
    mozjpeg "${backup}" > "$1"
    ;;
  "svg")
    optimize_svg "$@"
    ;;
  *)
    echo "Format .${extension} unsupported."
    exit 1
    ;;
esac